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Re: [Discuss-gnuradio] GSM spectrum: Invisible 200kHz carriers?


From: Alexander Chemeris
Subject: Re: [Discuss-gnuradio] GSM spectrum: Invisible 200kHz carriers?
Date: Sun, 5 Jun 2011 22:35:57 -0400

Hi,

This doesn't look like a GSM signal. There some pictures of GSM spectrum captured with USRP on the internet. I don't have a link near my hands, but you can probably google them easily.

Regards,
Alexander Chemeris.

On Jun 3, 2011 4:21 AM, "TheOperator" <address@hidden> wrote:
>
>
> Ben Wojtowicz wrote:
>> Are you saying that you are seeing 4MHz wide signals? Or are you seeing
>> ~200KHz wide signals seperated by 4MHz?I see about 200kHz wide signals
>> spaced 4MHz.
>
>
> Ben Wojtowicz wrote:
>> If you are seeing ~200KHz wide signals spaced by 4MHz, these most likely
>> are
>> GSM signals. The spec states that the spacing between adjacent channels
>> is
>> 200KHz, but that does not mean that an operator will place all of their
>> channels next to each other. In fact, it would be bad if they did this.
>> Keep in mind that the spacing between channels is 200KHz, but the occupied
>> bandwidth of a GSM signal is ~270KHz. This means that if GSM channels are
>> placed next to each other, ICI will increase significantly. Typically
>> what
>> is seen is a very sparse scattering of channels across the band.Thanks for
>> the explanation, this makes sense. The channels are not exactly 200kHz
>> wide however (see screenshots), but according to the frequencies, they
>> have to be GSM.
>
> I used this little system along with some GUI elements:
> http://old.nabble.com/file/p31763653/gsm-900-blocks.png
>
> The resulting spectrum looks like this:
> http://old.nabble.com/file/p31763653/gsm-900.png
>
> Such a peak has the following shape. Most of the power is distributed within
> ~50kHz. Do these smaller peaks have a special meaning?
> http://old.nabble.com/file/p31763653/gsm-900-zoomed.png
> --
> View this message in context: http://old.nabble.com/GSM-spectrum%3A-Invisible-200kHz-carriers--tp31761278p31763653.html
> Sent from the GnuRadio mailing list archive at Nabble.com.
>
>
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